1. Field of Invention
The present invention in a broad aspect relates to quantitative analysis of concentration of additives contained in a metal electroplating solution, and more specifically to a method for determining concentration of brightener and leveler in aqueous acid copper electroplating solutions.
2. Related Art
Traditionally, aluminum (Al) has been used as the material of choice for metalization in forming interconnect layers in the manufacture of semiconductor microelectronic integrated circuits. Al is commonly deposited on semiconductor structures by chemical vapor deposition (CVD), which allows for precise control and highly uniform deposition of the product metal-containing film.
Despite the prior ubiquity of Al as a metalization medium, performance demands associated with increasing signal speeds and decreasing feature geometries of microelectronics have exceeded the capabilities of Al metal. Copper (Cu) therefore is increasingly being utilized as a semiconductor interconnect metal. The properties of Cu are not amenable to conventional CVD metalization approaches, due in part to the lack of suitable copper source reagents, and in consequence Cu is typically deposited on the microelectronic device structure via electroplating.
Electroplating of copper, however, has various associated problems.
Generally, Cu is plated onto a substrate by electrolysis in an aqueous acid copper plating solution, which may for example comprise copper sulfate, sulfuric acid, and hydrochloric acid. The plating process with an unaugmented plating solution of such type normally proceeds too rapidly. The result of such plating rapidity is that previously formed vias, i.e., passages to lower-level structures, e.g., electrodes or other conductors or semiconductor regions in the microelectronic device structure, are bridged over, and not filled with Cu. Accordingly, the desired electrical path to the underlying structure is not formed, and the semiconductor device structure must be reworked or discarded.
In order to produce a brilliant copper finish on the plating surface of the microelectronic devices, various brighteners have to be added to the acidic electroplating solutions. Such brighteners usually comprise organic sulfonic and carboxylic acids, or their salts, and the use of such brighteners results in improved stability of the copper electroplating solutions and effective deposition of copper over a satisfactory current density range. For more details about suitable brighteners, see U.S. Pat. Nos. 2,707,166, 2,707,167, 2,830,014, 3,276,979, and 3,288,690.
The addition of brightener or brighteners alone, although resulting in a sufficiently brilliant finish, does not guarantee a smooth copper deposit layer on the plating surface of the microelectronic devices, if such plating surface contains small recesses or protrusions caused by microscopic cracks or scratches. In order to eliminate the effect of such microscopic cracks or scratches, levelers need to be further added to the copper electroplating solutions, which will give such electroplating solutions the xe2x80x9clevelingxe2x80x9d ability to produce relatively thicker layer of copper deposits in small recesses and relative thinner layer of copper deposits on small protrusions, thereby decreasing the depth of plating surface irregularities. Levelers known in the art include thiourea derivatives, condensation of thiourea with aliphatic aldehydes such as formaldehyde, and heterocyclic sulfur-nitrogen organic compounds, as disclosed in U.S. Pat. Nos. 3,101,305, 3,655,534, 3,725,220, 3,798,138, 3,972,789, and 4,038,161.
The speed of copper deposition and the quality and electrical and mechanical properties of the resulting copper deposits are critically dependent on the concentration of the additives such as brighteners and levelers. However, the concentration of these additives is not constant, due to either xe2x80x9cdrag-outxe2x80x9d by the microelectronic devices, or by electrochemical reaction and loss during the plating process. Therefore, precise real-time chemical concentration monitoring and control of the electroplating solutions are vital for producing high quality copper films or structures on microelectronic devices.
Precise monitoring of the concentration of the organic brighteners and levelers in a copper plating solution faces various difficulties. For example, the respective concentrations of the brighteners and levelers in the copper electroplating solutions are usually very low, e.g., part-per-million by volume (ppmv), which makes normal analytical procedures difficult to effectively apply, due to the masking effect of the high concentration of inorganic components such as copper sulfate and acids.
U.S. Pat. No. 6,280,602 discloses a method and apparatus for calculating the concentration of organic additives in a sample of metal plating solution, by measuring the plating potential of such solution and performing Pulsed Cyclic Galvanostatic Analysis on the measured plating potential. The content of such patent is incorporated by reference herein in its entirety for all purposes.
One problem that the U.S. Pat. No. 6,280,602 did not address is the interaction between the brightening effects of the brightener and the leveling effects of the leveler. The brightener usually accelerates the electroplating process, and the leveler, on the other hand, slows down the electroplating process. The cross-interference of the brightening effects and the leveling effects leads to inaccuracy of the concentration as calculated according to the method disclosed by U.S. Pat. No. 6,280,602.
It would therefore be a significant advance in the art, and is accordingly an object of the present invention, to provide a method for determining the concentration of the brightener and the leveler in an acid metal electroplating solutions, while reducing or minimizing the cross-interference between the effects of the brightener and the leveler, relative to the present state of the art.
Other objects and advantages will be more fully apparent from the ensuring disclosure and appended claims.
The present invention in a broad aspect relates to a method for determining concentration of brightener and leveler contained in an aqueous acid metal electroplating solution, comprising the steps of:
(a) providing a measuring apparatus that comprises:
(i) a reference electrode;
(ii) a rotating disc test electrode having a plating surface for depositing metal thereon and a rotational driver for rotating such rotating disc test electrode at a predetermined rotation speed, wherein such rotating disc electrode is disposed in a measurement chamber that comprises an electroplating current source electrode, and wherein the aqueous acid metal electroplating solution is introduced into the measurement chamber for measurement;
(iii) electroplating-driving electronics that are electrically and operatively coupled between the rotating disc test electrode and the electroplating current source electrode, whereby metal is selectively deposited onto the plating surface of the rotating disc test electrode from the aqueous acid metal electroplating solution in the measurement chamber at a constant known current density; and
(iv) measuring circuitry electrically and operatively coupled between the rotating disc test electrode and the reference electrode for measuring the electrical potential between the rotating disc test electrode and the reference electrode;
(b) determining the concentration of brightener in the aqueous acid metal electroplating solution, by using the measuring apparatus mentioned hereinabove to measure a first electrical potential of the metal electroplating solution at a first rotation speed and for a first electroplating duration, and by performing Pulsed Cyclic Galvanostatic Analysis of the first electrical potential as disclosed by U.S. Pat. No. 6,280,602; and
(c) determining the concentration of leveler in the aqueous acid metal electroplating solution, by using said measuring apparatus to measure a second electrical potential of said metal electroplating solution at a second rotation speed for a second electroplating duration, and by performing Pulsed Cyclic Galvanostatic Analysis of the second electrical potential, wherein the first rotation speed is lower than the second rotation speed, and
wherein the first electroplating duration is shorter than the second electroplating duration.
The present inventors have discovered that the brightening effect of the brightener is generally unaffected by and independent of the rotation speed of the rotating disc test electrode and the electroplating duration, while the leveling effect of the leveler is significantly increased at higher rotation speed and longer electroplating duration. Therefore, the cross-interference between the brightening effect and the leveling effect can be reduced or minimized, by firstly determining the concentration of the brightener at a lower rotation speed and/or for shorter electroplating duration, whereby the leveling effect of the leveler is less significant and therefore interferes less with the effect of the brightener, and then determining the concentration of the leveler at a higher rotation speed and/or for longer electroplating duration, wherein the leveling effect of the leveler is better manifested, while the brightening effect of the brightener remains approximately the same.
Preferably, the concentration of the brightener is first determined when the rotation speed of the rotating disc test electrode is set within a range of from about 0 rpm to about 4000 rpm, more preferably from about 0 rpm to about 2400 rpm, and/or the electroplating duration of the plating cycle is set within a range of from about 1 second to about 20 seconds, more preferably from about 1 second to about 10 seconds. The concentration of the leveler is then determined when the rotation speed of the rotating disc test electrode is preferably set within a range of from about 300 rpm to about 1250 rpm, more preferably from about 500 rpm to about 1250 rpm, and/or the electroplating duration is preferably set within a range of from about 1 second to about 25 seconds, more preferably from about 1 second to about 15 seconds.
Another aspect of the present invention involves a method for determining concentration of brightener and leveler contained in an aqueous acid metal electroplating solution, comprising the steps of:
(a) determining concentration of the brightener under a first set of measurement conditions;
(b) determining concentration of the leveler under a second set of measurement conditions, wherein the first set and second set of measurement conditions differ in at least one of the following measurement variables:
(i) rotation speed of a rotating disc electrode used for measuring plating potential of such aqueous acid metal electroplating solution; and
(ii) electroplating duration at which the plating potential of such aqueous acid metal electroplating solution is measured.
Preferably, the first set of measurement conditions includes a rotation speed of the rotating disc test electrode that is lower than that of the second set of measurement conditions, and/or an electroplating duration that is shorter than that of the second set of measurement conditions.
A further aspect of the present invention relates to a method for determining concentration of a first additive and a second additive contained in a metal electroplating solution, comprising the steps of:
(a) determining concentration of the first additive in the metal electroplating solution under a first set of measurement conditions;
(b) determining concentration of the second additive in the metal electroplating solution under a second set of measurement conditions,
wherein the first and second sets of measurement conditions differ from one another in at least one of the following measurement variables:
(i) rotation speed of a rotating disc electrode used for measuring plating potential of such metal electroplating solution; and
(ii) electroplating duration at which the plating potential of such metal electroplating solution is measured.
The first and second additives can be any additives that have interfering effects on the plating capability of the metal electroplating solution, including but not limited to brighteners and levelers.
Additional aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.